There's a trading group on facebook if you search for it. Also, you can buy singles on coolstuffinc.com and miniaturemarket.com. Generally, trade value is based on those sites and what they're selling for. Legendary for legendary or rare for rare is very common way to trade or multiple rares for a legendary.

Note: Finn can only use red "villian" "weapons" and "vehicles". Each of those words is important. You can't add a red hero vehicle card (unless you have a red hero) or a grey villian weapon to his deck for example.

While any trade that results in both parties getting something they want for something they want less is cool ... I'd generally advise against swapping legendaries for non-legendaries. The only exceptions I can think of are the very least sought after legendaries (AT-ST maybe) for the most highly sought after rares (Holdout Blaster, Jet Pack maybe). Even if a legendary sells for less as a single, it's just flat out more rare and would be harder to replace in a collection if you wanted it later.

Leia has previously been one of the lower priced rares >$5. I'm not sure where she's at now, but I definitely don't think she's worth any legendary on a 1:1 swap.

Of course, this is somewhat coloured by the fact I opened almost as many Leias as Rey's Staff and have been giving copies of her away to new players (I've given away at least six).

Maybe the excess copies I've opened corresponds with a shortage somewhere else in the world. Haha.